The waste gas, which pollutes the air, includes nitrogen oxides (NOx), sulfur oxides (SOx), carbon monoxide (CO), hydrocarbons (HCs), and particulate matters (PMs). Researches show that sulfur oxides in the waste gas emitted by motorcycles, automobiles, factories, power plants seriously affect human health. Therefore, the industry and academia have paid much effort to develop technologies for effectively eliminating sulfur oxides in waste gas.
The emission standard of motor vehicles is improving persistently. However, the continuously increasing motor vehicles still bring about more and more serious air pollution. In a motor vehicle, the engine thereof burns fuel and converts chemical energy into mechanical energy. The burning process of fuel generates exhaust gases, including nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbons (HCs), and particulate matters (PM), which would form photochemical smog, cause acid rain, damage the ecological environment and endanger human health.
Carbon monoxide comes from imperfect combustion. The capability of carbon monoxide to combine with hemoglobin to form carboxyhemoglobin (COHb) is 300 times higher than the capability of oxygen to combine with hemoglobin to form oxyhemoglobin (HbO2). Therefore, too high a concentration of carbon monoxide would degrade the capability of hemoglobin to transport oxygen. Nitrogen oxides are generated by the combination of nitrogen and oxygen and mainly in form of nitrogen monoxide (NO) and nitrogen dioxide (NO2). Under radiation of ultraviolet ray, nitrogen oxides are likely to react with hydrocarbons to form toxic photochemical smog, which has a special odor, irritates eyes, harms plants, and decreases atmospheric visibility. Further, nitrogen oxides react with humidity in the air to form nitric acid and nitrous acid, which are contributors to acid rain. Hydrocarbons may irritate the respiratory system even at a lower concentration and will affect the central nervous system at higher concentration. Besides, particulate matters may endanger health or even cause cancer.
Therefore, many nations, including EU, USA, Japan and Taiwan, have regulated stricter emission standards for nitrogen oxides, carbon monoxide, hydrocarbons (HCs) and particulate matters, such as (BINS of USA and EURO 6 of EU), which not only regulate the emission of harmful exhaust gases but also encourage the manufacturers to develop, fabricate or use the newest pollution control technologies and apparatuses.
A U.S. Pat. No. 5,401,372 disclosed an “Electrochemical Catalytic Reduction Cell for the Reduction of NOx in an O2-Containing Exhaust Emission”, which is dedicated to removing nitrogen oxides, wherein an electrochemical catalytic reducing reaction and a vanadium pentaoxide (V2O5) catalyst convert nitrogen oxides into nitrogen. However, the prior-art device needs an electric source to power an electrochemical cell. Therefore, the prior-art device consumes more power but cannot eliminate other harmful gases simultaneously.
A U.S. Pat. No. 9,028,764 disclosed an “Electro-Catalytic Honeycomb for Exhaust Emissions Control”, which can purify waste gas via eliminating nitrogen oxides, carbon monoxide, hydrocarbons (HCs) and particulate matters, wherein nitrogen oxides are decomposed into nitrogen and oxygen, and wherein carbon monoxide, hydrocarbons, and particulate matters are oxidized into water and carbon dioxide. The electro-catalytic honeycomb can eliminate several types of pollutants, neither consuming additional power nor using any reducing gas. However, the electro-catalytic honeycomb has higher fabrication cost. Besides, the electro-catalytic honeycomb is likely to be blocked while processing waste gas containing a higher concentration of particles.